Mineralogy and Petrology of Yamato 000593: Comparison with Other Martian Nakhlite Meteorites

Antarct. Meteorite Res., +0, -.῍/1, ,**- ῌ National Institute of Polar Research

Mineralogy and petrology of Yamato ***/3-: Comparison with other Martian nakhlite meteorites

Takashi Mikouchi, Eisuke Koizumi, Akira Monkawa, Yuji Ueda and Masamichi Miyamoto

Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo 1-chome, Bunkyo-ku, Tokyo ++-῍**--

Abstract: Yamato (Y) ***/3- is a new nakhlite recovered from Antarctica and is composed of roughly 2*ῌ augite, +*ῌ olivine and +*ῌ mesostasis. Augite is chemically homogeneous except for Fe-rich rims adjacent to the mesostasis. Olivine has more extensive chemical zoning, but the most Fe-rich part is also near the mesostasis. These observations suggest that chemical zoning of both augite and olivine was produced by interaction with the mesostasis. The crystallization history of Y***/3- as deduced from this study is as follows. (+) Crystallization of cumulus augite and olivine and formation of symplectites in olivine. (,) Accumulation of augite and olivine. (-) Mesostasis crystallization and interaction of the augite and olivine rims with the intercumulus melt. (.) Aqueous alteration. The petrography and mineralogy of Y***/3- is generally similar to other nakhlites, but minor mineralogical di#erences are observed. These di#erences resulted from di#erent thermal histories due to di#erent locations (burial depths) in the same cooling cumulate pile. Y***/3- is most similar to Nakhla and both samples experienced similar formation histories. However, later mesostasis crystallization of Y***/3- was more rapid than Nakhla due to its faster cooling rate. The burial depth of Y***/3- would be shallower than - m from the surface, and is intermediate between NWA2+1 and Nakhla. The abundance and mineralogy of the mesostasis as well as augite and olivine rim compositions are related to the burial depths of nakhlites.

key words: nakhlite, augite, olivine, crystallization, cooling rate

+. Introduction Among more than twenty known samples of Martian meteorites, six samples (Nakhla, Lafayette, Governador Valadares, Northwest Africa 2+1, Yamato ***/3-/ ***1.3/***2*, and Northwest Africa 332) show similar petrography and chemistry, constituting a “nakhlite” group( e.g., Bunch and Reid, +31/; Harvey and McSween, +33,a; Imae et al., ,**,a; Irving et al., ,**,; McSween, +33.; Meyer, +332; Sautter et al., ,**,). Nakhlites are cumulus clinopyroxenites with minor amounts of olivine and mesostasis. All the known nakhlites show very similar mineralogy, as well as similar crystallization and cosmic-ray exposure ages (+.- Ga and +*῍++ Ma, respectively) (e.g., Bogard et al., +32.; Bogard, +33/; Bunch and Reid, +31/; Eugster et al., +331; Harvey and McSween, +33,a; Marty et al., ,**+; Mikouchi and Miyamoto, +332, ,**,; Nakamura et al., +32,; Reid and Bunch, +31/; Sautter et al., ,**,; Shih et al., +330,

-. Mineralogy and petrology of the Y***/3- nakhlite 35

+332; Wadhwa and Crozaz, +33/). However, minor di#erences in mineralogy have been pointed out among nakhlite samples (e.g., Harvey and McSween, +33,a; Lentz et al., +333; Mikouchi and Miyamoto, +332, ,**,; Sautter et al., ,**,). It is believed that such mineralogical di#erences, albeit slight, are due to di#erent degrees of late magmatic and subsolidus atomic di#usion due to di#erent locations in the same cooling cumulate pile (e.g., Harvey and McSween, +33,a). Therefore, the discovery of new nakhlite samples could o#er substantial information to better understand the formation conditions and geological setting of this unique Martian meteorite group. Yamato ***/3-, Yamato ***1.3, and Yamato ***2*,, which are possibly paired, are the ﬁrst samples of nakhlites from Antarctica (Kojima and Imae, ,**+; Imae et al., ,**,a, b). Yamato ***/3- (hereafter, Y***/3-) is the largest achondrite sample in the Antarctic meteorite collections (Imae et al., ,**,c). In this paper, as a part of the consortium study of the Yamato nakhlites (Kojima et al., ,**,), we present mineralogy and petrology of Y***/3- to understand its formation conditions. We also compare the mineralogy of Y***/3- with those of other nakhlites to discuss the mineralogical relationships between Y***/3- and other nakhlites.

,. Samples and analytical techniques We studied three polished thin sections of Y***/3- (Y***/3-,0,--,Y***/3-,0,-0, and Y***/3-,02-+) supplied from the National Institute of Polar Research (NIPR). Back-scattered electron (BSE) images were taken with JEOL JXA2.* and Hitachi S- ./** (ﬁeld emission gun) scanning electron microscopes with energy dispersive spec- trometers (EDS), respectively (Dept. of Earth and Planet. Sci., University of Tokyo). X-ray elemental distribution maps were acquired by a JEOL JXA 23**Lelectron microprobe (Dept. of Earth and Planet. Sci., University of Tokyo). Accelerating voltage was +/ kV, and the beam current was 0*ῌ2* nA. Quantitative wavelength dispersive analyses were performed on a JEOL Superprobe 1-- electron microprobe (Ocean Research Institute, University of Tokyo) and a JEOL JCM 1-- mk II microprobe (Dept. of Earth and Planet. Sci., University of Tokyo) by using natural and synthetic standards. Microprobe analyses of most phases were obtained by focused beam at +/ kV accelerating voltage with a beam current of +, nA. For feldspar minerals in the mesostasis, a defocused beam of ῌ/ mm diameter was employed. The petrography and mineral compositions of Y***/3- were compared with those of other nakhlites by using data contained in Mikouchi and Miyamoto (+332) and Mikouchi and Miyamoto (,**,) for Nakhla, Governador Valadares and Lafayette and Mikouchi and Miyamoto (,**+) for Northwest Africa 2+1 (hereafter, NWA2+1). Because we have not analyzed Northwest Africa 332 and its available mineralogical data are limited at present, we do not consider this meteorite in this paper.

-. Petrography All the Y***/3- thin sections studied show a similar unbrecciated cumulus texture with the modal abundances of minerals, ῌ2*ῌ augite, +*ῌ olivine, and +*ῌ mesostasis (Fig. +). We did not observe any obvious di#erences in modal abundances among the 36 T. Mikouchi et al.

Fig. +a. Optical photomicrograph (open nikol) of the Y***/3- thin section (Y***/3-,02-+). The thin section shows a cumulate texture mainly composed of augite, olivine and the mesostasis. Augite shows a prismatic crystal texture up to +./ mm long.

Fig. +b. Back-scattered electron (BSE) image of the same ﬁeld of view as Fig +a. Bright phases are mostly olivines. Augite is princi- pally homogeneous except for the rims adjacent to the mesostasis. Aug: augite. Ol: olivine. Meso: mesostasis. three thin sections. Augite commonly occurs as euhedral to subhedral elongated grains whose longer dimension reaches up to +./ mm (Fig. +). Polysynthetic twinning is commonly observed. It is likely that augite is a cumulus phase as in other nakhlites, because of its high abundance and a preferred orientation (though not so obvious in our samples) of the longer dimensions of the grains. Minor amounts of low-Ca pyroxene are present (ῌ+ volῌ) and are usually associated with the mesostasis. Olivine grains are mostly anhedral and interstitial to cumulus augite (Fig. +b). The size of typical olivine grains is similar to that of pyroxene and they sometimes show euhedral crystal termination adjacent to the mesostasis. Small anhedral olivine grains (less than ,** mm in size) are also found and they are all interstitial to augite. Dark lamellar or patchy inclusions are commonly observed in large olivine grains (Fig. ,a). Mineralogy and petrology of the Y***/3- nakhlite 37

Fig. ,a. Optical photomicrograph (open nikol) of one of the largest olivine grains in Y***/3-.The presence of dark lamellar (or rectangular in this image) inclusions throughout the crystal is remarkable. Brown alteration products are observed around the rim and the fractures. Ol:olivine.

Fig. ,b. Optical photomicrograph (open nikol) of an altered olivine grain in Y***/3-. The presence of brown alteration products along the fractures is remarkable in this olivine grain. This olivine grain contains a poikilitic augite grain (Aug-p). The mesostasis (Meso) is also showing alteration similar to olivine. Ol:olivine. Aug:augite.

Fig. ,c Optical photomicrograph (open nikol) of the mesostasis area in Y***/3-. Note the pres- ·-� ence of brown alteration products interstitial to plagioclase laths. The mesostasis includes a Ti-rich magnetite grain (Ti-mag). Some of the tiny opaque grains in the mesostasis are Fe sulﬁde. : 100 µm Aug:augite. Ol:olivine. 38 T. Mikouchi et al.

The distributions of these lamellar inclusions within olivine grains are irregular unlike regular exsolution lamellae. Some large olivine grains poikilitically enclose rounded augite grains whose composition is identical to cumulus augite (Fig. ,b). Although magmatic inclusions are uncommon in olivine grains, single crystal augite inclusions, similar to those reported in Harvey and McSween (+33,b), are observed in some olivine grains. These inclusions are rounded with sizes up to ,** mm and are sometimes associated with K-feldspar. Olivine grains are pervasively altered to brown-colored, ﬁne-grained materials (smectite ?) along fractures (Fig. ,b) as reported in Imae et al. (,**,b) and Treiman and Goodrich (,**,). The mesostasis commonly displays a plumose texture, which is composed pre- dominantly of narrow laths of plagioclase with subordinate proportions of K-rich feldspar and a silica mineral (Fig. ,c). Plagioclase length reaches up to ,**ῌ-** mm although the width is only ῌ,* mm. Scattered lath-shaped grains of olivine and augite are also observed within the mesostasis and they are sometimes intergrown with plagioclase (Fig. ,c). Other minor phases in the mesostasis include Ti-rich magnetite (ulvo¨s pinel), ilmenite, Ca phosphate and Fe sulﬁde (pyrite ?). Ti-rich magnetite is up to -** mm in size although some grains are not associated with the mesostasis. The mesostasis also shows extensive secondary alteration indicated by the presence of brown rusty products as is seen in olivine (Imae et al., ,**,b; Treiman and Goodrich, ,**,) (Fig. ,c). Shock e#ects appear minor (or moderate) by judging optical and petrological properties of augite and olivine (e.g., slight undulating extinction of augite and olivine, polysynthetic twinning of augite).

.. Mineral compositions Representative mineral compositions of Y***/3- are given in Table +.

..+. Augite Most pyroxenes in Y***/3- are augite, having a large homogeneous core (average of +/- analyses: En-3 Fs,, Wo-3. Standard deviations for the En, Fs and Wo components are about +, respectively) (Figs. +b and -). The core composition is identical to that reported in Imae et al. (,**,a, b). The rims adjacent to the mesostasis show enrichment of Fe (Fig. +b), but the Wo content is almost uniform (En-3 Fs,, Wo-3ῌ En,* Fs.* Wo.*) except for the +*ῌ,* mm edge (En,* Fs.* Wo.*ῌ En+/ Fs1* Wo+/) (Fig. -). There is a sharp compositional boundary between the homogeneous core and the Fe-enriched rim as is clear from the high contrast BSE and Fe X-ray images (Figs. .aand/). At the boundary with the mesostasis, Fe-rich pyroxene displays a finger-like “overgrowth” texture (or crystallites that have nucleated on the pyroxene) intruding into the mesostasis and similar compositional pyroxene is observed within the mesostasis (Fig. .b). High-resolution BSE images show the presence of fine exsolution lamellae (ῌ/** nm wide) at the +*ῌ,* mm edges of cumulus pyroxenes adjacent to the mesostasis like other nakhlite pyroxenes (Mikouchi and Miyamoto, +332) (Fig. .c). The presence of fine exsolution lamellae is consistent with variable Wo content of pyroxene with constant En content (En: ῌ+/) at the rims (Fig. -). Similar fine exsolution lamellae are also Table +. Representative mineral compositions of major phases in Y***/3-.

Augite Augite Pigeonite Pigeonite Olivine Olivine Plagioclase K-feldspar Ti-magnetite Ilmenite Apatite "Rust"

(Core) (Rim) (Fe-rich) (Core) (Rim) Y***/3- the of petrology and Mineralogy

Si02 52.2 49.1 47.6 48.9 34.2 32.0 59.6 66.2 0.12 n.d. 0.65 43.6 Al203 0.60 1.93 0.74 0.10 n.d. 0.03 24.2 18.5 1.52 0.02 0.06 2.10 Ti02 0.18 0.48 0.35 0.05 n.d. 0.06 0.06 n.d. 15.3 49.1 n.d. 0.02 FeO 13.0 22.2 38.1 35.8 45.5 60.1 0.85 0.45 77.0 45.6 0.49 26.9 MnO 0.39 0.57 0.92 1.01 0.92 1.15 0.06 n.d. 0.38 0.66 0.12 0.46 MgO 13.6 7.04 3.96 9.49 18.6 6.42 0.02 n.d. 0.29 0.53 0.04 8.57 CaO 18.5 18.2 7.36 3.06 0.50 0.15 7.18 0.64 0.08 n.d. 53.5 1.60 Na,0 0.15 0.28 0.13 0.04 n.d. n.d. 7.56 3.92 n.d. n.d. n.d. 0.25

K20 0.02 n.d. n.d. n.d. n.d. n.d. 0.58 9.61 n.d. n.d. 0.03 0.43 Cr203 0.41 0.05 n.d. 0.02 n.d. n.d. n.d. n.d. 0.07 0.09 0.07 n.d. Vz03 0.07 0.02 n.d. 0.03 n.d. n.d. n.d. n.d. 0.29 0.22 n.d. n.d. NiO n.d. 0.02 n.d. n.d. 0.04 0.02 0.04 0.06 0.03 n.d. n.d. n.d. P20s 0.18 0.18 n.d. 0.06 n.d. n.d. 0.03 n.d. 0.03 n.d. 41.5 n.d. nakhlite Total 99.3 100.1 99.1 98.6 99.8 99.9 100.2 99.3 95.2 96.2 96.5 83.9

Fs 21.4 38.2 69.8 63.2 En 39.7 21.6 12.9 29.9 Wo 38.9 40.2 17.3 6.9

Fe#* 0.351 0.639 0.844 0.679 0.578 0.840

An 33.3 3.3 Ab 63.5 37.0 Or 3.2 59.7 39 *Fe#= Atomic Fe/(Fe+Mg). n.d. = not determined. 40 T. Mikouchi et al.

Ca Di Hd (CaMg) t (CaFe)

En (Mg)

Fig. -. Pyroxene quadrilateral of Y***/3- with olivine composition (dotted box along the Mg-Fe line). Pyroxene in Y***/3- is mainly augite with homogeneous composition (circled as “core”). The rims adjacent to the mesostasis shows Fe-enrichment (circled as “rim”). The extreme edges in contact with the mesostasis have very Fe-rich compositions with variable Wo content due to the presence of ﬁne exsolution (circled as “overgrowth”). Olivine composition has a wide range. The dotted box shows the compositional range observed in cumulus olivine grains. The arrow, “mesostasis”, shows more Fe-rich olivine compositions found in the mesostasis. observed in small pyroxene grains within the mesostasis (Fig. .d). The Al, O- content of augite ranges *..ῌ*.3 wtῌ (average of +/- analyses: *.0/ wtῌ. Standard deviation is *.+1) in the core (Fig. 0),showing heterogeneous distribution that is unrelated to the major element composition. The Al, O- content increases to ,.*ῌ,./ wtῌ towards the rim as atomic Fe/(FeῌMg) (fe#) increases from *.-/ to *.1 (Fig. 0). The Al, O- content suddenly drops down to *.0ῌ+.* wtῌ at the extreme edge (overgrowth) near the mesostasis (Fig. 0). The TiO, content exhibits a similar behavior,that is, TiO, is *.+ῌ *.- wtῌ in the core (average of +/- analyses: *.,+ wtῌ. Standard deviation is *.*0) and shows slight enrichment (TiO,῍*./ wtῌ) at the rim of fe#῍*.1 (Fig. 0). At the extreme edge (overgrowth) near the mesostasis,the TiO, content is *.-ῌ*.. wtῌ (Fig. 0). In contrast to Al and Ti, showing kink compositional trends,the Cr , O- content shows monotonous decrease from the core to the rim. In the core,the Cr , O- content is *.-ῌ*./ wtῌ (average of +/- analyses: *.-+ wtῌ. Standard deviation is *.*0)and drops down to nearly * at the rim of fe#῎*.0. Minor amounts of low-Ca pyroxene (pigeonite) are also present as small grains (ῌ,** mm) usually associated with the mesostasis. It is uniform in composition (En-* Fs0. Wo0) with Al, O- and TiO, contents that are both less than *.+ wtῌ.

..,. Olivine Large olivine grains show extensive chemical zoning unlike pyroxenes (Figs. / and 1). The most magnesian olivine in the sections studied is Fa/2 (Fig. -),which is found near the centers of large olivine grains (Fig. /). The zoned olivine grains have a very Fe-rich composition of Faῌ2/ at the edges adjacent to the mesostasis,while the rim Mineralogy and petrology of the Y***/3- nakhlite 41

Fig. .a. High contrast BSE image of olivine and augite grains in contact with the mesostasis in Y***/3-. Augite shows sharp compositional boundaries (shown by arrows) between the inner portions and the outer rim portions adjacent to the mesostasis. The olivine grain adjacent to the mesostasis has a well developed crystal termination. Aug: augite. Ol: olivine.

Fig. .b. BSE image of augite grains in contact with the mesostasis in Y***/3-. Augite grains show ﬁnger-like textures intruding into the mesostasis at their edges. Pyroxene grains with similar compositions are observed within the mesostasis. Aug: augite. compositions adjacent to cumulus augite are less Fe-rich (Fig. 1). Imae et al. (,**,a, b) also reported similar olivine compositions for Y***/3- and Y***1.3. The most ferroan composition of small olivine grains in the mesostasis reaches Fa3+. Chemical zoning is more remarkable in larger grains, and small grains (less than ῌ,** mm) are 42 T. Mikouchi et al.

Fig. .c. High resolution BSE image of Fe-rich pyroxenes at the cumulus pyroxene rims. Fine exsolution texture (thinner than /** mm wide) is clearly observed. Probably, darker lamellae correspond to Ca-rich pyroxene and brighter ones are Ca-poor pyroxene. Aug: augite. Pl: plagioclase in the mesostasis.

Fig. .d. High resolution BSE image of Fe-rich pyroxenes in the mesostasis. These two pyroxene grains also show the presence of ﬁne exsolution lamellae of similar size to that at the pyroxene edges (Fig. .c). Px: Fe-rich pyroxene in the mesostasis. Pl: plagioclase in the mesostasis. usually homogeneous (Fig. /). The olivine rim composition is similar to those of small grains with homogeneous composition. These observations suggest that chemical zoning can be attributed to cation exchange with the Fe-rich mesostasis. In some cases, olivine grains are intergrown with the mesostasis plagioclase. The CaO content of Mineralogy and petrology of the Y***/3- nakhlite 43

Low High Fig. /. Mg, Fe, and Ca X-ray maps of Y***/3-. The ﬁeld of view is 3 mm, yielding a slightly larger area than Fig. +. Yellow grains in the Mg map are cumulus augite. Olivine grains are colored from red to blue due to the presence of chemical zoning. The Fe map also indicates the presence of chemical zoning in olivine (grains showing the green cores to the red rims). Also note the presence of small Fe-rich homogeneous olivine grains. Augite grains (blue) show enrichment of Fe at the rims (core: dark blue, rim: bright blue). The Ca map shows that augite grains have almost uniform Ca content throughout the grains. The color of the mesostasis areas is mostly blue due to the presence of feldspar minerals. 44 T. Mikouchi et al.

----- 25 ----;;coo s------o - 1\Y:;00;0�5;93;-- 0 cP oAl203 0 O 0 2 0 19 O •Ti02 0 0 o o o ogi 0 0 0 � 0 9Roo cB 0 Core O O 0 o 98:R,ct, CP I % o o 'o o 0 oa<9 0 o o 'o _,..,o c.r 0 o O O 0 0 ,oa:P ' o 0 o O O ef' ° or 08 �o 0 0.5 �� i Af1• ./ • -..,;.:..��' it. • � .1'*'6 --.JA t • •::,. : ·��. ---=------=-----_J 0 0.3,;-�;--�----:-=--- 04. 0.5 0.6 0.7 0.8 0.9 Atomic Fe/(Fe+Mg)

Fig. 0. Al and Ti variations of augites in Y***/3- versus the fe# variation. Al shows the compositional range of Al, O-ῌ*..ῌ+.* wtῌ in the core, and then increases as fe# increases. The most Fe-rich augite has lower Al contents (Al, O-ῌ*./ῌ+.* wtῌ). The Ti content also has a cluster in the core (TiO,ῌ*.+ῌ*.- wtῌ) and then shows a similar behavior to Al. Ti increases as fe# increase, but it slightly decreases when fe# reaches *.0/. olivine is ῌ*./ wtῌ at the core and drops down to *.+ῌ*., wtῌ at the rim (Fig. 1). Brownish alteration products present along the olivine fractures (Fig. ,b) have low analytical totals (ῌ2/ wtῌ)(Table +) and are generally similar to “iddingsite” in other nakhlites (e.g., Imae et al., ,**,b; Treiman et al., +33-; Treiman and Goodrich, ,**,). They are Mg-poor and relatively enriched in Fe. The low analytical totals suggest the presence of water and ferric iron. However, they have variable compositions, and accurate chemical analysis was almost impossible. Symplectic inclusions are commonly observed in large olivine grains (Fig. ,a). They are usually parallel to one direction and /* mm long and less than / mm thick, but patchy inclusions are also observed (Figs. 2aand2b). The EDS analysis by FE-SEM suggests that they consist of magnetite (Ti-free) and augite like symplectites in Nakhla and Governador Valadares (Mikouchi et al., ,***). Similar symplectites are also observed at the grain boundaries of cumulus augite as patches ῌ+* mm in size (Fig. 2c). Augite inclusions within poikilitic olivine grains are also surrounded by these patches (Fig. 2d).

..-. Mesostasis The most abundant phase in the mesostasis is plagioclase, which is usually intergrown with silica or K-feldspar (glass ?) (Figs. ,cand3). The plagioclase composition is An-/ Ab0, Or-ῌAn,* Ab1. Or0 (Fig. +*) similar to that reported in Imae et al. (,**,a) and (,**,b). These plagioclase grains show clear birefringence under the optical microscope, suggesting that they have not transformed into “maskelynite” by shock. K-feldspar has a wide range of composition and the most K-rich feldspar has an Mineralogy and petrology of the Y***/3- nakhlite 45

0.8 �------�

0.75 Augite �------·- 0.7 p � 0.65

0.6

0.55 Y000593 olivine 0.5 '-----�------� 0 200 400 600 800 1000 1200 1400 Distance (micrometer) 7 ,------, 0. _ 0.6 . ...,,...... 0.5 •,. ·---- . . :...... , ....·.. .-. ·. � . -. . .. · ... � 0.4 ,..·-...... u ...... 1ffe. 0 . . . � 3 �---·--·--- 0.2 Augite .-----·--·> Mesostasis 0.1 Y000593 olivine 0 '----�------' 0 200 400 600 800 1000 1200 1400 Distance (micrometer)

Fig. 1. The chemical zoning proﬁles of the atomic Fe/(FeῌMg) content (Fa content) and Ca content of olivine in Y***/3-. The dotted arrows show where the olivine grain is in contact with augite and the mesostasis. It is obvious that the rim adjacent to the mesostasis shows more developed zoning patterns in both Fa and Ca. This olivine grain has a core composition of Fa/2 . The Ca zoning is observed at the rim (the area of +***ῌ+,** mm) whereas Fa zoning penetrates further into the grain (the area of 0**ῌ+,** mm). This could be due to di#erent atomic di#usion rates between Fe-Mg and Ca (e.g., Misener, +31.; Jurewicz and Watson, +322). orthoclase content of Or1/ (Fig. +*). The optical microscopic analysis shows the presence of abundant rusty brown alteration products in the mesostasis as is seen in olivine grains (Fig. ,c). The chemical composition is generally similar to that in olivine. Ca phosphate in the mesostasis is Cl-apatite as the EDS analysis shows the presence of abundant Cl.

.... Opaques Most opaque phases are present within or associated with the mesostasis and/or olivine. Ti-rich magnetite (ulvo¨s pinel) is the most common opaque phase in Y***/3-. It has a slight variation in chemical composition, ranging +ῌ- wtῌ Al, O-, +-ῌ+2 wtῌ 46 T. Mikouchi et al.

Fig. 2a. BSE images of an olivine grain in Y***/3- including abundant symplectic inclusions (indicated white arrows in the above image). These symplectic inclusions are composed of augite (darkarea) and magnetite (bright area). Ol: olivine. Aug: augite. Mag: magnetite.

,ῌ TiO,, and 1+ῌ11 wtῌ FeO (all Fe is calculated as Fe ) (Fig. ++). Because the analytical totals are 3-ῌ30 wtῌ (Table +), signiﬁcant amounts of Fe-ῌ are probably present. Some Ti-rich magnetite grains include exsolution lamellae of ilmenite up to / mm wide (Fig. +,). Isolated grains of ilmenite were also observed. Fe sulﬁdes are mostly S-rich, probably pyrite rather than pyrrhotite.

/. Crystallization history of Y***/3- There is no doubt that Y***/3- is a cumulate rock like other nakhlites (e.g., Harvey and McSween, +33,a; Wadhwa and Crozaz, +33/; Lentz et al., +333). Augite is clearly a cumulus phase, but it is not obvious whether olivine is cumulus or not. However, the interaction with the mesostasis is seen in both augite and olivine, causing Fe enrichment at their edges adjacent to the mesostasis. Thus, we believe that olivine is also a cumulus Mineralogy and petrology of the Y***/3- nakhlite 47

Fig. 2b. BSE image of the irregu- larly-shaped patch showing a symplectic texture in a Y***/3- olivine grain. This patch is also composed of augite (dark area) and magnetite (bright area). Ol: olivine. Aug: augite. Mag: magnetite.

Fig. 2c. BSE image showing the boundary between cumulus augite (Aug) and olivine (Ol) in Y***/3-. The arrows show the presence of patches having the symplectic textures similar to Figs. 2aand2b.

Fig. 2d. BSE image showing the patches with symplectic textures along the grain boundaries between poikilitic augite (Aug) and the host olivine grain (Ol) in Y***/3-. 48 T. Mikouchi et al.

Fig. 3. BSE images of the mesostasis area in Y***/3-. The mesostasis is mainly composed of lath-shaped plagioclase (Pl) intergrown with silica (Si). Bright lath-shaped grains in the mesostasis are olivine and pyroxene. phase, although its crystallization followed augite crystallization because of the interstitial texture of olivine to augite. However, there is a possibility that olivine cores have nucleated concurrently with the pyroxene and then continued growing (or renewed growth) once pyroxene ceased and accumulation occurred. According to those textur- al observations, the sequence of the Y***/3- crystallization is thus augite, olivine, and the mesostasis. More detailed crystallization history of Y***/3- is as follows (Fig. +-). At ﬁrst augite crystallized from a magma and olivine crystallization followed. Both augite and olivine show chemical zoning in Y***/3-. However, augite has a large homogeneous core and the chemical zoning is only seen in the rim area. In contrast, chemical zoning of olivine is much more extensive. This can be explained by faster Mineralogy and petrology of the Y***/3- nakhlite 49

Ab An

Fig. +*. Major element compositions (Ca, Na and K) of feldspars in Y***/3-. Plagioclase has the composition of An-/ Ab0, Or-ῌ An,* Ab1. Or0 . K-feldspar has a wide range of composition, reaching Or1/ .

2Ti

Al Cr

Fig. ++. Major element compositions (Cr, Al and Ti) of spinels in Y***/3-. Spinels in Y***/3- are Ti-rich magnetite (ulvo¨spinel). The dashed circles show the compositions of spinels in shergottites, which are quite di#erent from those of Y***/3-. atomic di#usion of olivine relative to pyroxene (e.g., Misener, +31.; Fujino et al., +33*). Because Fe enrichment was observed near the rims adjacent to the mesostasis and the degree of chemical zoning was larger in olivine than pyroxene, chemical zoning would 50 T. Mikouchi et al.

Fig. +,. BSE image of the mesostasis area including opaque minerals in Y***/3-. Ti-rich magnetite grain in this image shows ilmenite exsolution lamellae parallel to three orientations (probably parallel to (+++)). An individual ilmenite grain is also present. Ti-mag: Ti-rich magnetite. Ilm: ilmenite. Ol: olivine. Aug: augite. Meso: mesostasis. be produced by interaction with the Fe-rich intercumulus melt. This scenario has been proposed by Harvey and McSween (+33,a) and we believe that this is also applicable for the case of Y***/3-. Before accumulation of augite and olivine, both minerals would be homogenized. The formation of symplectite exsolution in olivine would also occur at this stage due to relatively slow cooling. The next process included formation of a cumulus network in the magma, producing interstitial melt to both augite and olivine and modifying homogeneous compositions into zoned ones at the rims of augite and olivine. We also suggest that “overgrowth” of low-Ca pyroxene at the extreme edges of cumulus augite occurred at this stage. Feldspar minerals and Ti-rich magnetite crystallized in the intercumulus melt, formingthe mesostasis. Our thin sections do not give deﬁnite information of the origin of “rust” in olivine and the mesostasis. However, Treiman and Goodrich (,**,) reported that at least some of this material is of Martian origin as other nakhlites contain pre-terrestrial alteration products. Imae et al. (,**,b) analyzed Xe and Kr isotopes of the weathering products and also suggested their Martian origin. Therefore, the ﬁnal process that a#ected Y***/3- was aqueous alteration on Mars forming“rust” in olivine and the mesostasis.

0. Comparisons with other nakhlites 0.+. Petrography The texture of Y***/3- is generally similar to all other nakhlites (e.g., Harvey and McSween +33,a; Lentz et al., +333; Mikouchi and Miyamoto, ,**+; Sautter et al., ,**,). All the nakhlites have a similar ratios of modal proportions of augite and olivine (augite/olivineῌca. .ῌ0)(e.g., Lentz et al., +333; Sautter et al., ,**,). The Mineralogy and petrology of the Y***/3- nakhlite 51

/ Olivine crystallization

Homoge'!1za· tion and symplect,te form ation in olivme

lntercumulus melt

overgrowth

Feldspar Mag Mesostas,s. crystallization Alteration Feldspar Mag

Fig. +-. Schematic illustration showing the formation history of Y***/3-. (a) Augite crystallization from a magma. Aug: augite. (b) Olivine crystallization from the same or di#erent magma. Ol: olivine. (c) Homogenization of chemical composition of both augite and olivine. Symplectite formation occurred at this stage. Symp: symplectite. (d) Accumulation of augite and olivine occurred, forming a cumulate network with intercumulus melt. (e) The mesostasis rapidly crystallized and chemical zoning was produced at the rims of augite (not extensive) and olivine adjacent to the mesostasis. “Overgrowth” of Fe-rich pyroxene was also formed. Mag: magnetite. (f) Aqueous alteration a#ected olivine and the mesostasis, forming brown “rusty” products (“Rust”). 52 T. Mikouchi et al.

Table ,. Mesostasis abundance and related mineralogical characteristics of nakhlites.

Sample Me sos tasis Plagioclase Olivine composition Pyroxene composition name abundance size at the rim at the rim

NWA817 -20vo1% <5 µm wide Fa,;, Atomic Fe/(Mg+Mg) = 0.75 Y000593 -10vo1% -20 µmwide Fa.2 Atomic Fe/(Mg+Mg) = 0. 71

Nakhla -5vo1% -50 µmwide Fa,2 Atomic Fe/(Mg+Mg) = 0.66 Governador Valadares -5vo1% -50 µmwide Fa,o Atomic Fe/(Mg+Mg) = 0.66

Lafayette -5vo1% -50 µmwide Fa68 Atomic Fe/(Mg+Mg) = 0.50 grain sizes of cumulus augite and olivine in Y***/3- are also very similar (average *./ῌ +.* mm long with a maximum of +./ mm long) to those of other nakhlites (e.g., Lentz et al., +333; Sautter et al., ,**,). The most notable petrographic di#erence between Y***/3- and other nakhlites is the abundance of the mesostasis (Table ,). NWA2+1 has similar (or slightly higher) modal abundance of the mesostasis, but the radiating plagioclase is rare (or very thin) in NWA2+1 (Mikouchi and Miyamoto, ,**+; Sautter et al., ,**,). The mesostasis in NWA2+1 is rather composed of Si-rich feldspathic glass with skeletal Ti-rich magnetite, suggesting a faster cooling of the mesostasis (Mikouchi and Miyamoto, ,**+; Sautter et al., ,**,). The mesostasis mineralogy of Y***/3- is rather similar to Nakhla, Governador Valadares and Lafayette with the presence of radiating plagioclase laths. However, plagioclase in Y***/3- is thinner (ῌ,* mm wide) compared to those in Nakhla, Governador Valadares and Lafayette (ῌ/* mm) (Table ,). Bunch and Reid (+31/) and Friedman et al. (+332) made similar observations on the plagioclase textures in the nakhlite mesostasis. The di#erent sizes of plagioclase in the mesostasis suggest that the cooling rate of Y***/3- was faster than those of other nakhlites, perhaps due to a shallower location in the igneous body (Harvey and McSween, +33,a; Mikouchi and Miyamoto, ,**,; Mikouchi et al., ,**,). The di#erence in the mesostasis abundance between Y***/3-/NWA2+1 and Nakhla/ Governador Valadares/Lafayette suggests that NWA2+1 and Y***/3- formed at locations in a cooling cumulus pile where the abundances of cumulus phases were lower than other nakhlites. The abundance of the mesostasis or the degree of “packing” of cumulus phases may be related to the burial depths of nakhlites (Table ,). The di#erent mesostasis mineralogy also relates to other mineralogical characteristics as discussed in the following chapter and also reported in Imae et al. (,**,b). The degree of aqueous alteration of Y***/3- is also generally similar to those of other nakhlites (e.g., Treiman et al., +33-). This suggests that ﬂuid activity was uniform throughout the nakhlite igneous body.

0.,. Augite mineralogy The chemical composition of the augite core in Y***/3- is nearly identical to those of other nakhlites, suggesting that they have a similar petrogenesis (e.g., Harvey and McSween, +33,a; Wadhwa and Crozaz, +33/) (Figs. - and +.). However, augite grains in Y***/3- have a distinctive rim composition di#erent from any other nakhlite Mineralogy and petrology of the Y***/3- nakhlite 53

Ca Di Hd (CaMg) (CaFe) ,.--.,.,.....--..--t---, ---.,-----..

En Fs ( Fe) Mg) Olivine Fa50 (

Fa mo L.I_ _._ _ _._ _ Fa ioo _.__�NNaakkhkla�v:�z�W,�z�W,�:?;fW,u.z:£.W,u.z:£.?)d,._;....___JI GV §Lafayette

Fig. +.. Pyroxene quadrilateral of Y***/3- along with the pyroxene compositions of Nakhla, Governador Valadares (GV), Lafayette and NWA2+1. Their olivine compositions (Fa and Fo contents) are also shown. Augite cores of all nakhlites have an identical composition although the rim compositions are di#erent. Olivine compositions of nakhlites have variations in both core and rim compositions. pyroxenes (Figs. - and +.). Nakhla and Governador Valadares are the most similar in terms of the pyroxene rim compositions. Pyroxenes in Nakhla and Governador Valadares have Fe-rich compositions with variable Wo contents (but constant En) similar to Y***/3- due to the presence of ﬁne exsolution lamellae (Mikouchi and Miyamoto, +332). Nevertheless, the most Fe-rich composition of Y***/3- pyroxene is more Fe-rich than those of Nakhla and Governador Valadares. NWA2+1 does not contain Fe-rich augite with variable Wo content in our sample (Mikouchi and Miyamoto, ,**+), but Sautter et al. (,**,) reported the presence of very Fe-rich pyroxenes with variable Wo content. These Fe-rich pyroxenes are more Fe-rich than that of Y***/3- (Fig. +.). Lafayette is quite di#erent from other nakhlites including Y***/3- in its homogeneous composition except for the extreme edge adjacent to the mesostasis. This homogeneous composition is probably due to slower cooling. Thus, compositions of Y***/3- pyroxenes are intermediate between those of Nakhla/ Governador Valadares and NWA2+1.

0.-. Olivine mineralogy All the nakhlites including Y***/3- have unique olivine compositions (Figs. - and +.). Olivine composition is more variable than pyroxene in nakhlites (Fig. /). This is probably because olivine has faster atomic di#usion rates than those of pyroxene (e.g., Misener, +31.; Fujino et al., +33*) and its composition is sensitive to minor di#erences in thermal history. The most magnesian composition of Y***/3- olivine (Fa/2)is identical to that of Nakhla, but Y***/3- olivine extends to more ferroan compositions (Fa2,) than Nakhla (Fa12). NWA2+1 olivine also has Fe-rich compositions of Faῌ2/, 54 T. Mikouchi et al.

but the most magnesian olivine is more Mg-rich (Fa/.). As is seen in augite composition, Lafayette olivine is almost completely homogeneous (Fa02). In this sense, Lafay- ette appears to have been located at the deepest position in the cooling cumulus pile, causing homogenization of its mineral compositions (e.g., Harvey and McSween, +33,a). The rim compositions of Nakhla and Governador Valadares olivines are very similar (Fa1*ῌ1,) to each other. NWA2+1 and Y***/3- olivines have more Fe-rich olivine rim compositions of Fa3, and Fa2,, respectively (Mikouchi and Miyamoto, ,**+). The Fe-rich compositions of NWA2+1 and Y***/3- olivine rims are consistent with high abundances of their mesostases. These data suggest that the mesostasis crystallization of NWA2+1 and Y***/3- occurred very near the Martian surface. Y***/3- provides several pieces of evidence (e.g., plagioclase size in the mesostasis, olivine composition) that it was located slightly deeper than NWA2+1. The ﬁnal crystallization of Nakhla and Governador Valadares would occur the locations intermediate between those of Y***/3- and Lafayette. Thus, nakhlite olivines are a good indicator of the cooling history of nakhlites. In a previous study, we estimated cooling rates of olivines from Nakhla, Lafayette, Governador Valadares and NWA2+1 by using chemical zoning of Fe-Mg and Ca at their olivine rims adjacent to the mesostases (Mikouchi and Miyamoto, ,**,). We computed the Fa and Ca zoning proﬁles at various cooling rates from ++**῍Cto 1**῍C after we assumed that initial olivine compositions are homogeneous (Fa/., CaO: *./. wtῌ). We revised the Fe-Mg di#usion rate of Misener (+31.) by dependence of oxygen fugacity and Fe/Mg ratios of olivine (Miyamoto et al., ,**,). In this calcula- tion, we set the oxygen fugacity at the QFM bu#er that is the estimated redox condition of nakhlites (e.g., Reid and Bunch, +31/). Ca di#usion data are from Jurewicz and Watson (+322). The obtained cooling rates correspond to a burial depth ranging from ῎+ m for NWA2+1 to ῌ-* mforLafayette. Nakhla and Governador Valadares are intermediate between these two (-ῌ+* m). Because Y***/3- appears to have been located intermediate between NWA2+1 and Nakhla/Governador Valadares, the burial depth of Y***/3- would be shallower than - m from the surface. Olivine may have started cooling from higher temperature than ++**῍C. In that case, olivine should have experienced faster cooling rates than the result obtained for the ++**ῌ1**῍C temperature range. If olivine started cooling from +***῍C, the obtained cooling rates would be about one order of magnitude slower than those from ++**῍C. Even if this is the case, the burial depth for NWA2+1 is shallower than + m. Lafayette has the slightly deeper burial depth. The presence of symplectic inclusions in olivine has previously been observed only in Nakhla and Governador Valadares (Mikouchi and Miyamoto, +332; Mikouchi et al., ,***). This is consistent with the similarity in olivine compositions between Y***/3- and Nakhla/Governador Valadares. The reason for the absence of such symplectites in NWA2+1 and Lafayette is unclear. Perhaps, relatively fast cooling of the cumulus augite and olivine in NWA2+1 (corresponding to aῌbῌc stages of Fig. +-) prevented exsolution of symplectites. Lafayette is distinct from other nakhlites in many respects and it may have a di#erent history from other nakhlites. Mineralogy and petrology of the Y***/3- nakhlite 55

0... Opaque mineralogy The chemical composition of Ti-rich magnetite and ilmenite in Y***/3- is generally similar to those of other nakhlites, suggesting that all nakhlites formed under similar redox conditions near the QFM bu#er (e.g., Reid and Bunch, +31/). We have not performed a detailed analysis on sulﬁdes, but the presence of pyrite has been reported from other nakhlites (e.g., Bunch and Reid, +31/).

1. Conclusions There is no doubt that Y***/3- is a new nakhlite (not paired with any other previously known nakhlites)since it was found in Antarctica, thousands of kilometers away from other nakhlites. These rocks, including Y***/3-, share similar mineralogy and ages, suggesting that they came from the same igneous body on Mars and were ejected by the same impact event (e.g., Bogard et al., +32.; Bogard, +33/; Bunch and Reid, +31/; Eugster et al., +331; Harvey and McSween, +33,a; Marty et al., ,**+; Imae et al., ,**,a, b; Lentz et al., +333; Mikouchi and Miyamoto, +332, ,**,; Mikouchi et al., ,**,; Nakamura et al., +32,, ,**,; Okazaki et al., ,**,; Oura et al., ,**,; Reid and Bunch, +31/; Sautter et al., ,**,; Shih et al., +330, +332, ,**,; Ueda et al., ,**,; Wadhwa and Crozaz, +33/; Yamashita et al., ,**,). In other words, it is very notable that we have nakhlite samples from a variety of terrestrial locations that show slightly di#erent mineralogy from each other. The di#erence might be due to di#erent thermal histories due to di#erent locations (burial depths)in the same cooling cumulate pile ( e. g., Harvey and McSween, +33,a; Imae et al., ,**,b; Lentz et al., +333; Mikouchi and Miyamoto, +332, ,**,; Mikouchi et al., ,**,; Wadhwa and Crozaz, +33/).Y***/3- is most similar to Nakhla and the formation of cumulus phases occurs under the same conditions (Fig. +-). The similarity between Y***/3- and Nakhla is also found in reﬂectance spectra (Ueda et al., ,**,). Later mesostasis crystallization of Y***/3- was more rapid than Nakhla due to its faster cooling rate, and produced minor di#erences between them. We previously estimated that the burial depths of Nakhla and NWA2+1 were .ῌ/ mand*./ m, respectively, according to the Fa zoning proﬁles of olivine (Mikouchi and Miyamoto, ,**,). This study suggests that Y***/3- would be located between NWA2+1 and Nakhla. The abundance of the mesostasis or the degree of “packing” of cumulus phases may be related to the burial depths of nakhlites (Imae et al., ,**,b).

Acknowledgments

The Y***/3- samples were kindly allocated by NIPR as a part of the consortium study. We thank Drs. N. Imae, K. Misawa, and H. Kojima for useful discussions and Drs. P.C. Buchanan and R.C.F. Lentz for their constructive reviews of the manuscript. We are indebted to Mr. O. Tachikawa and Mr. H. Yoshida for their technical assistance during electron beam analyses. The electron microscopy was performed in the Elec- tron Microbeam Analysis Facility for mineralogy at Department of Earth and Planetary Science, University of Tokyo. 56 T. Mikouchi et al.

References

Bogard, D.D. (+33/): Exposure-age-initiating events for Martian meteorites: Three or four? Lunar and Planetary Science XXVI. Houston, Lunar Planet. Inst., +.-ῌ+... Bogard, D.D., Nyquist, L.E. and Johnson, P. (+32.): Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites. Geochim. Cosmochim. Acta, .2, +1,-ῌ+1-3. Bunch, T.E. and Reid, A.M. (+31/): The nakhlites I: Petrography and mineral chemistry. Meteoritics, +*, -*-ῌ-+/. Eugster, O., Weigel, A. and Polnau, E. (+331): Ejection times of Martian meteorites. Geochim. Cosmochim. Acta, 0+, ,1.3ῌ,1/1. Friedman, R.C., Taylor, G.J. and Treiman, A.H. (+332): Nakhlites and Theo’s flow: Formation of extrusive pyroxenites. Lunar and Planetary Science XXIX. Houston, Lunar Planet. Inst., Abstract #++3* (CD-ROM). Fujino, K., Naohara, H. and Momoi, H. (+33*): Direct determination of cation di#usion coe$cients in pyroxene. EOS, 1+, 3.-. Harvey, R.P. and McSween, H.Y., Jr. (+33,a): Petrogenesis of the nakhlite meteorites - Evidence from cumulate mineral zoning. Geochim. Cosmochim. Acta, /0, +0//ῌ+00-. Harvey, R.P. and McSween, H.Y., Jr. (+33,b): The parent magma of the nakhlite meteorites: Clues from melt inclusions. Earth Planet. Sci. Lett., +++, .01ῌ.2,. Imae, N., Okazaki, R., Kojima, H. and Nagao, K (,**,a): The first nakhlite from Antarctica. Lunar and Planetary Science XXXIII. Houston, Lunar Planet. Inst., Abstract #+.2- (CD-ROM). Imae, N., Ikeda, Y., Shinoda, K., Kojima, H. and Iwata, N. (,**,b): Two nakhlites from Antarctica: Y ***/3- and Y***1.3. Antarctic Meteorites XXVII. Tokyo, NatlInst. PolarRes., ./ῌ.1. Imae, N., Iwata, N. and Shimoda, Y. (,**,c): Search for Antarctic meteorites in the bare ice field around the Yamato Mountains by JARE-.+. Antarct. Meteorite Res., +/, +ῌ,.. Irving, A.J., Kuehner, S.M., Rumble, D. III, Carlson, R.W., Hupe´, A.C. and Hupe´, G.M. (,**,): Petrology and isotopic composition of orthopyroxene-bearing nakhlite NWA 332. Meteorit. Planet. Sci., -1, A1*. Jurewicz, A.J.G. and Watson, E.B. (+322): Cations in olivine, part ,:Di#usion in olivine xenocrysts, with application to petrology and mineral physics. Contrib. Mineral. Petrol., 33, +20ῌ,*+. Kojima, H. and Imae, N., ed. (,**+): Meteorite Newsletter, +*,(,), 3 p. Kojima, H., Nakamura, N., Imae, N. and Misawa, K. (,**,): The Yamato nakhlite consortium. Antarctic Meteorites XXVII. Tokyo, NatlInst. PolarRes., 00ῌ02. Lentz, R.C.F., Taylor, G.J. and Treiman, A.H. (+333): Formation of a martian pyroxenite: A comparative study of the nakhlite meteorites and Theo’s Flow. Meteorit. Planet. Sci., -., 3+3ῌ3-,. Marty, B., Marti, K. and the Theodore Monod Consortium (,**+): Noble gases in new SNC meteorites NWA 2+1 and NWA .2*. Meteorit. Planet. Sci., -0,A+,,ῌA+,-. McSween, H.Y., Jr. (+33.): What we have learned about Mars from SNC meteorites. Meteoritics, ,3, 1/1ῌ 113. Meyer, C. (+332): Mars Meteorite Compendiumῌ,**+. Houston, NASA Johnson Space Center ,-1 p. (http: //www/curator.jsc.nasa.gov/curator/antmet/mmc/mmc.htm). Mikouchi, T. and Miyamoto, M. (+332): Pyroxene and olivine microstructures in nakhlite Martian meteorites: Implication for their thermal history. Lunar and Planetary Science XXIX. Houston, Lunar Planet. Inst., Abstract #+/1. (CD-ROM). Mikouchi, T. and Miyamoto, M. (,**+): Northwest Africa 2+1: A new nakhlite similar to others but distinct. Meteorit. Planet. Sci., -0,A+-.. Mikouchi, T. and Miyamoto, M. (,**,): Comparative cooling rates of nakhlites as inferred from iron- magnesium and calcium zoning of olivines. Lunar and Planetary Science XXXIII. Houston, Lunar Planet. Inst., Abstract #+-.- (CD-ROM). Mikouchi, T., Yamada, I. and Miyamoto, M. (,***): Symplectic exsolution in olivine from the Nakhla martian meteorite. Meteorit. Planet. Sci., -/, 3-1ῌ3.,. Mikouchi, T., Koizumi, E., Monkawa, A., Ueda, Y. and Miyamoto, M. (,**,): Comparative mineralogy of the new nakhlite Yamato ***/3- with other nakhlite Martian meteorites. Antarctic Meteorites Mineralogy and petrology of the Y***/3- nakhlite 57

XXVII. Tokyo, Natl Inst. Polar Res., 2-ῌ2/. Misener, D.J. (+31.): Cation di#usion in olivine to +.**ῌCand-/ kbar. Geochemical Transport and Kinetics, ed. by A.W. Ho#mann et al. Washington D.C., Carnegie Inst. of Washington, ++1ῌ+,3. Miyamoto, M., Mikouchi, T. and Arai, T. (,**,) Comparison of Fe-Mg interdi#usion coe$cients in olivine. Antarct. Meteorite Res., +/, +.-ῌ+/+. Nakamura, N., Unruh, D.M., Tatsumoto, M. and Hutchison, R. (+32,): Origin and evolution of the Nakhla meteorite inferred from the Sm-Nd and U-Pb systematics and REE, Ba, Sr, Rb and K abundances. Geochim. Cosmochim. Acta, .0, +///ῌ+/1-. Nakamura, N., Yamakawa, A., Yamashita, K., Kobayashi, T., Imae, N., Misawa, K. and Kojima, H. (,**,): REE abundances and Rb-Sr age of a new Antarctic nakhlite Yamato ***/3-. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., ++,ῌ++.. Okazaki, R., Nagao, K., Imae, N. and Kojima, H. (,**,): Noble gases in Antarctic nakhlites, Yamato ***/3- and Yamato ***1.3. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +-.ῌ+-0. Oura, Y., Shirai, N. and Ebihara, M. (,**,): Chemical composition of Y***/3- and Y***1.3. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +.-ῌ+./. Reid, A.M. and Bunch, T.E. (+31/): The nakhlites part II: Where, when and how. Meteoritics, +*, -+1ῌ-,.. Sautter, V., Barrat, J.A., Jambon, A., Lorand, J.P., Gillet, Ph. and Javoy, M. (,**,): A new Martian meteorite from Morocco: the nakhlite North West Africa 2+1. Earth Planet. Sci. Lett., +3/, ,,-ῌ ,-2. Shih, C.-Y., Nyquist, L.E. and Wiesmann, H. (+330): Sm-Nd systematics of nakhlite Governador Valadares. Lunar and Planetary Science XXVII. Houston, Lunar Planet. Inst., ++31ῌ++32. Shih, C.-Y., Nyquist, L.E., Reese, Y. and Wiesmann, H. (+332): The chronology of the nakhlite, Lafayette: Rb-Sr and Sm-Nd isotopic ages. Lunar and Planetary Science XXIX. Houston, Lunar Planet. Inst., Abstract #++./ (CD-ROM). Shih, C.-Y., Wiesmann, H., Nyquist, L.E. and Misawa, K. (,**,): Crystallization age of Antarctic nakhlite Y***/3-: Further evidence of nakhlite launch pairing. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +/+ῌ+/-. Treiman, A.H. and Goodrich, C.A. (,**,): Pre-terrestrial aqueous alteration of the Y-***1.3 nakhlite meteorite. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +00ῌ+01. Treiman, A.H., Barrett, R.A. and Gooding, J.L. (+33-): Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics, ,2, 20ῌ31. Ueda, Y., Mikouchi, T., Miyamoto, M. and Hiroi, T. (,**,): First analysis of the reﬂectance spectrum of Yamato ***/3-: The spectroscopic similarity between Yamato ***/3- and Nakhla. Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +1+ῌ+1-. Wadhwa, M. and Crozaz, G. (+33/): Trace and minor elements in minerals of nakhlites and Chassigny: Clues to their petrogenesis. Geochim. Cosmochim. Acta, /3, -0,3ῌ-0./. Yamashita, K., Nakamura, N., Imae, N., Misawa, K. and Kojima, H. (,**,): Pb isotopic signature of Martian meteorite Yamato ***/3- (A preliminary report). Antarctic Meteorites XXVII. Tokyo, Natl Inst. Polar Res., +2*ῌ+2,. (Received November +-, ,**,; Revised manuscript accepted February -, ,**-)